By Yuka Obayashi and Mari Saito; Editing by Michael Perry10 October 2013

TOKYO (Reuters) – Radiation levels in seawater just outside one of the damaged Fukushima reactors spiked this week to the highest level in two years, the operator of the crippled Japanese nuclear plant said on Thursday.

Radiation levels on Wednesday, the day six workers were exposed to highly radioactive water, jumped 13 times the previous day's reading, the highest levels since late 2011.

A massive quake and tsunami hit the power station, operated by Tokyo Electric Power Co, also known as Tepco, in March 2011, causing three reactor meltdowns and hydrogen explosions.

Tepco, which is pouring hundreds of tonnes of water to keep reactors cool, has struggled to contain the build up of radioactive water at the plant.

In the latest incident, a worker on Wednesday mistakenly detached a pipe connected to a treatment system, releasing seven tonnes of highly radioactive water.

The accidents at the Fukushima Daiichi plant, 220 km (130 miles) north of Tokyo, are adding to the crisis and stirring doubt over Tepco's abilities to carry out a complex cleanup widely expected to take decades.

Tepco said combined Cesium-134 and Cesium-137 readings just outside the damaged No. 2 reactor spiked to 1,200 becquerels per litre on Wednesday, more than 13 times the level on Tuesday.

Cesium-134 readings were 370 becquerels per litre while Cesium-137 was 830/litre within a silt fence right outside the reactor building. Regulatory limits for Cesium, which emits a strong gamma radiation and is harmful to the human body, is 90 bq/litre for Cesium-137 and 60 bq/litre for Cesium-134.

A Tepco spokesman said the sudden spike in radiation was caused by construction work near the No. 2 building.

Workers are injecting chemicals to harden the ground on the seaside of the Fukushima reactor buildings to prevent contaminated water from flowing out to the ocean. The pressure from pumping chemicals into the ground pushed some contaminated soil out into the port area, the spokesman said.

Tepco also said Cesium-137 readings just outside the silt fence next to the No.2 reactor rose to 160 bq/litre, also above the regulatory limit and almost double the previous day's level.

The readings were taken right next to the Fukushima plant but hundreds of meters from the port entrance that connects to the Pacific Ocean. [more]